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RationaleThe molecular mechanism(s) by which mechanical ventilation disrupts alveolar development, a hallmark of bronchopulmonary dysplasia, is unknown.ObjectiveTo determine the effect of 24 h of mechanical ventilation on lung cell cycle regulators, cell proliferation and alveolar formation in newborn rats.MethodsSeven-day old rats were ventilated with room air for 8, 12 and 24 h using relatively moderate tidal volumes (8.5 mL.kg−1).Measurement and Main ResultsVentilation for 24 h (h) decreased the number of elastin-positive secondary crests and increased the mean linear intercept, indicating arrest of alveolar development. Proliferation (assessed by BrdU incorporation) was halved after 12 h of ventilation and completely arrested after 24 h. Cyclin D1 and E1 mRNA and protein levels were decreased after 8–24 h of ventilation, while that of p27Kip1 was significantly increased. Mechanical ventilation for 24 h also increased levels of p57Kip2, decreased that of p16INK4a, while the levels of p21Waf/Cip1 and p15INK4b were unchanged. Increased p27Kip1 expression coincided with reduced phosphorylation of p27Kip1 at Thr157, Thr187 and Thr198 (p<0.05), thereby promoting its nuclear localization. Similar -but more rapid- changes in cell cycle regulators were noted when 7-day rats were ventilated with high tidal volume (40 mL.kg−1) and when fetal lung epithelial cells were subjected to a continuous (17% elongation) cyclic stretch.ConclusionThis is the first demonstration that prolonged (24 h) of mechanical ventilation causes cell cycle arrest in newborn rat lungs; the arrest occurs in G1 and is caused by increased expression and nuclear localization of Cdk inhibitor proteins (p27Kip1, p57Kip2) from the Kip family.

Herein, we determined the contribution of mechanical ventilation, hyperoxia and inflammation, individually or combined, to the cytokine/chemokine response of the neonatal lung. Eight-dayold rats were ventilated for 8 h with low (ϳ3.5 mL/kg), moderate (ϳ12.5 mL/kg), or high (ϳ25 mL/kg) tidal volumes (V T ) and the cytokine/chemokine response was measured. Next, we tested whether low-V T ventilation with 50% oxygen or a preexisting inflammation induced by lipopolysaccharide (LPS) would modify this response. High-, moderate-, and low-V T ventilation significantly elevated CXCL-2 and IL-6 mRNA levels. Low-V T ventilation with 50% oxygen significantly increased IL-6 and CXCL-2 expression versus low-V T ventilation alone. LPS pretreatment combined with low-V T ventilation with 50% oxygen amplified IL-6 mRNA expression when compared with low V T alone or low V T ϩ 50% O 2 treatment. In contrast, low V T up-regulated CXCL-2 levels were reduced to nonventilated levels when LPS-treated newborn rats were ventilated with 50% oxygen. Thus, low-V T ventilation triggers the expression of acute phase cytokines and CXC chemokines in newborn rat lung, which is amplified by oxygen but not by a preexisting inflammation. Depending on the individual cytokine or chemokine, the combination of both oxygen and inflammation intensifies or abrogates the low V T -induced inflammatory response. (Pediatr Res 68: 63-69, 2010) B ronchopulmonary dysplasia (BPD) remains the most important cause of respiratory morbidity in very low birth weight infants. Mechanical ventilation (MV), intra-uterine infections and oxidative stress up-regulate proinflammatory cytokines/chemokines including IL-1␤, IL-6, and IL-8 (1). Elevated concentrations of these cytokines/chemokines in amniotic fluid and bronchoalveolar lavage fluid (BALF) have been associated with BPD (2,3). The contribution of each risk factor, alone or combined, to the inflammatory response remains to be determined.Ample animal studies have suggested that high frequency oscillatory ventilation (HFOV) is less injurious compared with conventional ventilation (CMV) (4,5). However, in the baboon model of BPD impaired alveolarization and capillary development occurred in spite of appropriate oxygenation and use of HFOV (4). MV with moderate and high tidal volumes increased lung cytokine/chemokine response to systemic endotoxin in rabbits (6) and newborn rats (7). Oxidant injury alone can produce the pathologic features of BPD (8). Inflammatory cells such as monocytes and neutrophils are primary contributors to the oxygen-induced lung injury (9,10). Other animal studies have investigated the contributions of oxygen exposure and MV alone or in combination. In term ventilated piglets hyperoxia caused less lung damage than hyperoxia combined with hyperventilation but more than hyperventilation alone (11). Premature baboons ventilated with the minimum necessary supplemental oxygen had significant less damage than those ventilated with 100% oxygen (12), but alveolarization and capillary developmen...